Nasturtiums -- give a nice kick to salads and make a beautiful edible decoration for chocolate cake.
Lavender -- good in shortbread and scones. Divine in lavender and lemon cheesecake.
Poppies -- OK, the seeds not the flowers. Sprinkle on muesli. Bake on bread. Don't eat the seeds from the yellow poppies (California poppy), just the red ones.